Friction stir welding of dissimilar metals

ABSTRACT

When a friction stir weld tool penetrates the interface of two workpieces of dissimilar metal alloy materials, the resultant weld of the different alloy materials may produce a weak weld joint. Such weak joints are often experienced, for example, when attempting to form spot welds or other friction stir welds between a magnesium alloy sheet or strip and an aluminum alloy sheet or strip. It is discovered that suitable coating compositions including an adhesive placed at the interface of assembled workpieces can alter the composition of the friction stir weld material and strengthen the resulting bond. In the example of friction stir welds between magnesium alloy and aluminum alloy workpieces, it is found that combinations of an adhesive with copper, tin, zinc, and/or other powders can strengthen the magnesium-containing and aluminum-containing friction stir weld material.

This application is a division of U.S. patent application Ser. No.12/418,663, filed on Apr. 6, 2009, and titled “Friction Stir WeldingUsing an Adhesive, Copper, Tin and Zinc Interlayer”, which is acontinuation-in-part of U.S. patent application Ser. No. 12/250,750filed on Oct. 14, 2008, and titled “Friction Stir Welding of DissimilarMetals.”

TECHNICAL FIELD

This invention pertains to the use of friction stir welding in joiningdissimilar metal members, such as a magnesium alloy panel and analuminum alloy reinforcing piece. More specifically, this inventionpertains to the placement of an interlayer material such as a mixture ofadhesive and metallic powder between facing surfaces of the differentmetal composition members for incorporation into the joint materialproduced by the friction stir weld tool to increase the strength of thewelded joint.

BACKGROUND OF THE INVENTION

There are manufacturing applications in which it could be useful to weldmembers of dissimilar metal compositions to fabricate, for example,relatively light-weight articles. For example, in the manufacture ofautomotive vehicle body parts it might be desired to bond an aluminumalloy reinforcing strut to a magnesium alloy panel. Often, suchdissimilar metal members are difficult to join by conventional joiningtechniques such as fusion welding processes because they form massive,brittle intermetallic compositions that weaken the joint. It iscontemplated that such dissimilar metal parts might be joined usingfriction stir welding practices.

In friction stir welding a rotating tool with an axial probe andshoulder is pressed into a surface of an assembly of metal workpieces.The rotating probe and shoulder engage the workpieces at a welding site.The frictional heat and continued pressure on the probe and shouldertemporarily soften, plasticize, and mix material in engaged portions ofthe workpieces. When the rotating tool is pressed generallyperpendicularly into a spot on the workpieces and then retracted, afriction stir spot weld is formed. The friction stir tool may beretracted and moved and successively engaged along the surface of one ormore workpieces to form a series of friction stir spot welds. When therotating tool is pressed into a workpiece surface and moved in thesurface, a friction stir linear weld or seam weld may be formed.Similarly, the friction stir tool may be moved along an interface ofabutting edges of two or more workpieces to form a friction stir buttweld. Collectively, these various weld patterns are referred to asfriction stir welding (FSW). FSW may include friction stir spot welding(FSSW).

Where the composition of the metal pieces to be joined yields a suitableweld zone, good joint strengths may be obtained. When some dissimilarmetals are joined with FSW, the formation of brittle, low melting pointintermetallic materials in the weld zone may yield weak or brittle weldbonds. This may happen when, for example, it is desired to join amagnesium alloy member to an aluminum alloy part.

It is an object of this invention to provide a method of achievingstrong friction stir weld bonds between workpieces of dissimilar metalcompositions such as, for example, between magnesium alloy workpiecesand aluminum alloy workpieces.

SUMMARY OF THE INVENTION

Practices of this invention are useful in friction stir weldingsituations in which dissimilar metal workpieces are to be joined and therespective compositions of the workpieces fail to yield good bondstrengths by conventional friction stir welding techniques. For example,friction stir plasticized aluminum and magnesium alloys may form a lowmelting temperature composition that weakens an intended weld. Duringfriction stir welding of aluminum to magnesium, the temperature of theweld site may be high enough to produce a low melting Al—Mg eutecticliquid. This liquid not only limits the size of the stir zone but alsotends to stick to the friction stir welding tool when the tool iswithdrawn from the weld site. The formation of such a liquid materialproduces a weak bond between the aluminum and magnesium work pieces.

The joint strength of a friction stir spot weld depends on the size ofits stir zone that was formed during welding. When friction stirplasticization of an interface comprising elements of two dissimilarmetal members fails to produce a good friction stir bond, it may bebeneficial to change the composition of the friction stirred zone byadding one or more interlayer materials comprising, for example, anadhesive mixed with metal powders and/or non-metal powders atinterface(s) of the workpieces to be joined.

The adhesive may be used as a vehicle for the metal and/or non-metalpowder to be applied to the joint for FSW. The adhesive may incorporatethe metal and/or non-metal powder and provide better adherence of thepowders to surfaces that will be subjected to FSW. The adhesive andpowder mixture may be applied at and/or around the weld site. Inaddition, the adhesive that is not burned off during the FSW process maybe cured during the manufacturing process, increasing the strength ofthe joint, for example from about 750 lb to about 1500 lb lap shearstrength. This additional strength may arise from the additional bondingarea created by the adhesive. In one embodiment, the adhesive underand/or adjacent the FSSW pin tool will burn off leaving the powder toreact with the parent metals. Adhesive remote from the pin tool willcure at some point in the manufacturing process, adding strength to thejoint.

In embodiments of the invention where an aluminum member is to be joinedto a magnesium member, intended weld sites may be provided with a layerincluding a mixture of (a) adhesive and copper and tin powders, or (b)adhesive and copper, tin and zinc powders, or (c) adhesive and zincpowder, or (d) adhesive and other suitable metallic and/or non-metallicpowder compositions and mixtures comprising, for example but not limitedto, at least one of aluminum, magnesium, silicon, strontium, cerium (orother lanthanoids), silver, titanium, antimony, nickel, chromium,manganese, iron, vanadium, niobium, zirconium, yttrium, molybdenum,tungsten, brass, bronze, steels, carbon, alumina, magnesia, silica,titanium oxide or iron oxides, or combinations thereof. Examples ofadhesives that may be used in the mixture include, but are not limitedto, at least one of epoxies, polyurethanes, acrylics, tape adhesive, orspray adhesives. The adhesive may be in any suitable form, for examplebut not limited to a liquid, spray, mist, paste, or particles.

The adhesive and the metal powders may be added in separate layers of asingle component or as a layer of multi-component mixture. Such acomposition is applied as a suitable coating or interlayer tointerfacial surfaces of the parts to be welded. Then the parts areassembled and supported for friction stir welding. These coatings mayalso be applied onto the top surface of the workpiece facing thefriction stir welding tool. During the welding, the added materials arestirred, mixed, and may react with adjacent aluminum and magnesium inthe stir-affected zone. The resulting, more complex mixture forms astronger weld bond. The coating may also improve corrosion properties ofthe weld joint.

Such adhesive and powder compositions are chosen by experience orexperiment for improving the mechanical properties of the FSW. Forexample, the adhesive and powder composition may react with the parentmetals (e.g., aluminum alloy and magnesium alloy) to form constituentsof higher melting temperatures (higher than those of the constituentsthat may form from the parent metal interactions alone) in the stir zoneor increase the viscosity of the intermetallic liquid produced such thatthe stir zone becomes relatively solid or firm and decreases itstendency to stick to the weld tool. After welding, any remainingadhesive may cure and strengthen the bond between the adjacentworkpieces. The added powder materials may react with the parent metalsto form other microstructural constituents. The curing of the adhesiveand an increase in melting temperature of the stir zone material and/oran increase in the stir zone firmness with a dispersion of smallparticles of added powder material and/or reaction products may increasethe strength and/or toughness of the resulting joint between thedissimilar metal workpieces.

In another embodiment of the invention that is complementary to the useof interface-composition changing powders, a high thermal conductivityanvil is used to support the workpieces against the friction stir tooland to promote heat transfer from the stir zone to minimize formation oflow melting point intermetallic materials during friction stir welding.The increased cooling rate is used to avoid or minimize melting in theweld region. The increased cooling rate is used to minimize the amountformed of low melting temperature intermetallic materials and toincrease the firmness of the resultant mixture of metals andintermetallic liquid.

As stated above, the composition-changing powder material may bedeveloped and specified by experience or experiment. For example, thetemperature in the stir zone during friction stir welding of aluminumand magnesium workpieces can easily be 450° C. and above. Tin and zinchave relatively low melting temperatures, approximately 232° C. and 420°C., respectively. Therefore, during friction stir welding, tin and zincare melted and the tin or zinc liquid can react with the adjacentaluminum and magnesium materials. For example, tin can react withmagnesium to form a mixture of solid Mg₂Sn (melting temperature of about770.5° C.) particles and tin-rich Mg—Sn liquid during friction stirwelding. And the adhesive that is not burned off during the FSW processcures and contributes to the bond between the aluminum and magnesiumworkpieces. In the meantime, aluminum and magnesium can form an Al—Mgeutectic liquid. The Mg₂Sn particles thus formed and the added particlessuch as copper particles along with the inclusion particles that existedwithin the parent materials mix with the Al—Mg eutectic liquid todecrease its fluidity and increase its firmness. This mixture furthermixes with the un-reacted aluminum and magnesium parent materials in thestir-affected zone resulting in a relatively firm and strong stir zone.This firmness also decreased the tendency for the stir zone material tostick to the weld tool. Upon cooling, a strong and tough weld is formedof a complicated composite of aluminum alloy, magnesium alloy, Mg₂Sn,Al—Mg intermetallic compound like Al₃Mg₂, and copper. It may alsocontain some tin.

In one embodiment, the interlayer material composition is used in theform of a powder or the like to facilitate adhesive bonding anddispersion in, and alloying with, the friction stir tool plasticizedmetal from the adjacent facing workpieces. The supplemental coatingmaterial is applied to the contacting regions of overlapping or abuttingworkpieces of different metal compositions. The coating material may beplaced as loose powder on facing surfaces of one or both of the piecesbefore they are assembled and supported for FSW. The addition ofinterlayer material may be done by any suitable coating method like coldspray, electron beam vacuum deposition, thermal spray, etc., or bycladding or simply by adding a thin piece of material of suitablecomposition, in addition to application as loose powders.

Other objects and advantages of the invention will be apparent from adetailed description of various embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the lap shear strength of FSSW joints withinterlayer compositions of adhesive-metal powder mixtures and metalpowders without adhesive.

FIG. 2 illustrates the formation of an in-line sequence of friction stirspot welds in the surface of overlapping edges of, for example, a firstmetal alloy workpiece with a second metal alloy workpiece. A frictionstir weld tool is illustrated in a withdrawn position poised for theformation of a third spot weld. A coating of mixed adhesive and powderedmaterial has been placed between the overlapping surfaces along the pathof intended spot welds.

FIG. 3 illustrates a cross-sectional view of a single spot weld andadjacent region formed in the FIG. 2 assembly, showing the deformedregions of the first and second metal alloy pieces with the stir zoneand nearby coating material.

FIG. 4 illustrates abutting pieces of dissimilar metal strips or plateswith a layer of coating material of adhesive and alloying powders placedbetween the abutting surfaces.

FIG. 5 illustrates a friction stir tool in the process of forming acontinuous friction stir butt weld seam between the abutting metalpieces of FIG. 4.

FIG. 6 illustrates friction stir welding of overlapped sheets ofdifferent base alloy compositions with a thin interlayer of adhesive andmetal powders placed between the metal sheets along the weld path.

DESCRIPTION OF PREFERRED EMBODIMENTS

Friction stir welding of dissimilar metals, for example aluminum alloyto magnesium alloy workpieces, often causes the formation of a fairlylarge amount of brittle, low melting point intermetallic phases, whichis undesirable for attaining high joint strengths. Melting in FSWoperations may cause the stir zone material to stick to the pin tool andthereby only low joint strengths are achieved.

In various examples, friction stir spot welding of 1.6 mm thick, AA5754aluminum alloy strips to 1.3 mm thick, AZ31 magnesium alloy strips wasconducted. The pieces were supported on a steel anvil. A friction stirtool having a probe height of about 2.4 mm, a probe diameter of about 3mm and a tool shoulder diameter of about 10 mm was rotated at a speed of1600 rpm and applied to the aluminum surface at a force of about 8 kN.The probe had a threaded external surface. The probe penetrated throughthe aluminum strip and into the magnesium strip. The plasticized spotweld was formed in a few seconds and the tool and probe retracted. Aftera spot weld was formed the sheets were subjected to a shear load to testthe strength provided to the joined pieces by the single spot weld. Alap shear strength value of only about ninety pounds was obtained. Whilethe melting points of the respective strips were above 600° C.,magnesium and aluminum are known to form eutectic compositions that meltmore than 150° C. lower. It appears that such brittle, low melting pointcompositions formed during friction stir welding and led to the weaknessof the spot weld.

It has been found that much higher spot weld strength values can beobtained by introducing, for example, a mixture of copper and tin powderparticles, or a mixture of copper, tin, and zinc powders, or zincparticles between the aluminum and magnesium work pieces. These mixturesmay also improve corrosion properties of the weld joint. The addition ofan adhesive to the powder particles further increases the spot weldstrength values. The adhesive provides a vehicle for applying thepowders to the joint to be welded. The portion of the adhesive that isnot burned off during welding forms an adhesively bonded joint. Theaddition of the adhesive thus may increase the total joint from about700 lb to about 1500 lb.

Lap shear strength of the friction stir spot welded joints of 1.6 mmAA5754 aluminum to 1.3 mm AZ31 magnesium using copper-tin powderinterlayer materials, with copper weight fraction varying from 0.1 to0.9, was improved to 200 to 450 lb from about 90 lb for those weldswithout the copper-tin interlayers. The powder mixture with a copperfraction of 0.25 gave the 450 lb lap shear strength. In anotherembodiment where a strip of 1 3 mm AZ31 magnesium sheet (placed on thetop, i.e., on the tool side) is friction stir spot welded to a strip of2.5 mm AA5754 sheet with a steel anvil, a lap shear strength of about200 lb was obtained without any coating additions.

With the use of an interlayer of zinc powder, a lap shear strength ofabout 420 lb was obtained. With the use of an interlayer of adhesive and25 wt % Cu, 75 wt % Sn metal powder, along with a copper anvil, a lapshear strength of as high as 1500 lb was obtained for a 1.6 mm AA5754aluminum strip welded to a 1.3 mm AZ31 magnesium strip. FSW joints madewith the adhesive and metal powder mixture resulted in a 20-50% increasein strength when compared to joints made with addition of the metalpowders only without the adhesive.

In other trials, friction stir spot welds were formed on overlappingaluminum and magnesium strips while they were supported on a highthermal conductivity copper anvil. The high thermal conductivity anvilwas sized and shaped for quickly conducting excess heat (causingmelting) from the friction stir spot weld region of the lower of theworkpieces which was pressed against the copper anvil. Three metalpowder compositions comprising, by weight, one part copper to threeparts tin, one part each of copper, tin, and zinc (designatedhereinafter as copper-tin-zinc), and 100% zinc were found to markedlyincrease the lap shear strength of a friction stir spot weld formedbetween the aluminum and magnesium alloy strips.

In a series of tests, coatings of mixed copper, tin, and zinc particleswere applied to the aluminum strips by a cold spray coating procedure toa thickness of about 0.2 mm Cold spray may be performed by using asupersonic carrier gas to propel metal powders toward the substrate tobe coated. The high speed particles impact the substrate and deform intoa dense and adherent coating. The gas temperature in the spray nozzle isbelow the melting temperature of the particles. With the complementaryuse of a copper anvil, lap shear joint strengths above 750 lb wereobtained for FSSW joints of the 1.6 mm thick, AA5754 aluminum to 1.3 mmthick, AZ31 magnesium. For example, the coating addition of one parteach of copper, tin, and zinc gave an average lap shear strength of 600lb, 100% zinc, 650 lb, and one part copper to three parts tin, 750 lb.The use of a copper anvil and/or water-cooled anvil, for example awater-cooled steel anvil, reduces the temperature of the stir zoneduring welding by dissipating excess generated heat and helps tomaintain a solid or relatively firm stir zone.

FSSW trials also were conducted with copper-anvil supported 1.6 mm thickAA5754 aluminum to 1.3 mm thick AZ31 magnesium using other powdermixtures, such as 10Cu-90Sn (500 lb), 25Ag-75Sn (500 lb), 25Ag-65Sn-10Zn(615 lb), Zn (650 lb), 10C-90Sn (500 lb), Al₂O₃ (550 lb), and50A1-50Al₂O₃ (606 lb) (where the compositions are given in weightpercent), compared with a lap shear strength of up to 250 lb without theaddition of any coating or powder mixtures. The compositions areindicated in weight percentage with the average lap shear strengthsgiven in the parentheses following each powder mixture. There are otherpowder mixtures that also improved joint strength significantly, e.g.,an Al₂O₃ and 25 wt % Cu-75 wt % Sn powder mixture (approximately equalvolume fractions) gave an average lap shear strength of 695 lb. Thepowder additions can also be made to the top surface (i.e., on thefriction stir tool side) or top surface and faying surfaces. Forexample, a FSSW of 1.3 mm AZ31 sheet to a 2.5 mm AA5754 aluminum sheetwith aluminum powders on top of the AZ 31 sheet and copper-tin-zincpowders at faying surfaces gave a lap shear strength of 580 lb.

FSSW trials were also performed with copper-anvil supported 1.6 mm thickAA5754 aluminum to 1.3 mm thick AZ31 magnesium using interlayercompositions with and without adhesive. FIG. 1 shows the lap shearstrength of the FSSW joints from these trials and of a joint bondedwithout FSSW versus bottom thickness (in millimeters), the final depthof the welds. The diamond data points are for FSSW joints with adhesiveand metal-powder mixture. The circle data points are for FSSW jointswith a 25 wt % Cu-75 wt % Sn coating applied by the cold spray coatingprocedure to a thickness of about 0.2 mm The triangle data points arefor FSSW joints with a Cu—Sn—Zn (equal weight percentage) coatingapplied by the cold spray coating procedure to a thickness of about 0.1mm. The square data points are for joints bonded without FSSW operationwhere adhesive and metal-powder mixture was applied to the joints. Asevidenced by the diamond data points, the addition of the adhesivesignificantly increases the strength of the FSSW joint. This is becausethe adhesive that is not burned off during the FSSW process cures andincreases the joint strength.

Further practices of friction stir welding with powder coatings will bedescribed.

In FIG. 2, an edge 14 of a strip 10 (or sheet or plate or otherworkpiece shape) of a first metal composition overlaps an edge 16 of asecond strip 12 (or sheet or plate or other workpiece shape) of a secondmetal composition. By way of illustration, the first metal compositionmay be an aluminum alloy and the second metal composition may be amagnesium alloy. Lower face 18 of upper strip 10 lies against upper face20 of strip 12. In this embodiment the overlapping edges 14, 16 of therespective strips are parallel and it is intended to form a series offriction stir spot welds in a line between the parallel edges 14, 16. Acoating layer or interlayer composition 22 of a compositionpredetermined to improve the strength of the spot welds was applied to asurface of at least one of the strips 10, 12 before they were assembledin the illustrated overlapping position. In this example, coating layer22 is applied in a generally rectangular strip (solid edge and dashedlines in FIG. 2) between the facing surfaces 18, 20 of sheets 10, 12.Coating layer 22 extends along the path of the intended spot welds. Theinterlayer composition 22 may include adhesive and metallic and/ornon-metallic powder compositions or mixtures. In an embodiment in whichthe metal parts are formed respectively of a magnesium alloy and analuminum alloy, interlayer compositions such as those described abovemay be used in the coatings.

The overlapping strips 10, 12 are assembled and supported against theapplied force of a friction stir tool 24. In various embodiments of theinvention, the workpieces 10, 12 are supported on a high thermalconductivity anvil as is illustrated in FIG. 6 and described more fullyin connection with that figure. The supporting anvil is not illustratedin FIG. 2. Friction stir tool 24 has a round cylindrical body 26 mergedwith a concentric truncated conical tip 28 and a threaded axial probe30. The probe may also be conical in shape. The threads on probe 30 maybe replaced by stepped spirals or other suitable profiles that promotefriction stirring and the formation of a strong weld. The bottom face oftruncated conical tip provides an annular shoulder 32 from which theaxial-extending probe 30 extends. The probe 30 and shoulder 32 arerotated and pressed into engagement with a predetermined friction stircontacting surface of workpieces in friction stir welding. As is known,shoulder 32 and probe 30 can be separately actuated and rotate atdifferent speeds, the mechanisms of which are not described herein. Inthe embodiment illustrated in FIG. 2, spot welding sites on uppersurface 34 of workpiece strip 10 are the designated contact regions forprobe 30 and shoulder 32 of friction stir tool 24.

In friction stir welding operations, friction stir tool 24 is securelyheld in a powered friction stir machine, not illustrated, that isadapted to locate the tool probe 30 and annular shoulder 32 against oneor more surfaces of a workpiece or workpieces. In FIG. 2 friction stirtool 24 is positioned in an attitude with the rotational axis 36 of thetool, including probe 30, aligned generally perpendicular to a spot weldsite (indicated by cross-mark 38 in FIG. 2). The friction stir machineis adapted to rotate friction stir tool 24 as indicated by therotational arrow. The friction stir machine forcefully advances the tool(lowers the tool in FIG. 2 per downward directional arrow) so that therotating probe 30 and shoulder 32 first engage surface 34 of strip 10,and penetrate through strip 10 into strip 12. As will be described morefully with respect to FIG. 3, the frictional contact between therotating probe 30 and shoulder 32 and the materials of the respectiveworkpieces generates intense local heating. The engaged material isplasticized. After a brief period of such friction stirring, the tool 24is temporarily retracted from contact with the workpieces. Theplasticized or stirred metal hardens to form a spot weld (e.g., spotweld sites 40, 42 in FIG. 2), and tool 24 advances to a next frictionstir spot weld position, such as over site 38. Spot weld sites 40 and 42reflect the penetration of threaded probe 30 and engagement of thesurface 34 of strip 10 with the shoulder 32 of tool 24. In this examplethe rotational speed of tool 24 is 1600 rpm as it is pressed into theworkpieces with a force of 8 kN. The probe may penetrate about 2.5 mmthrough strip 10 and into strip 12.

FIG. 3 is a schematic (and not necessarily to scale), cross-sectionalview of a friction stir spot weld site, such as the region of frictionstir spot weld site 40, in FIG. 2. In FIG. 3, fragmentary portions ofupper strip 10 and lower strip 12 are seen. Generally conical hole 44remains at the spot weld site after the upward extraction of tool 24which lifts probe 30 and shoulder 32 from their penetration into theworkpieces. Hole 44 extends through the affected portion of strip 10 andthrough about 50% or more of the thickness of strip 12. In the case ofthis spot weld, an annular mass of hardened stirred material 46 locallyjoins strips 10, 12 in the spot weld. Stepped spiral indentations 47from probe 30 are seen in the stirred material 46. A thin layer ofunconsumed interlayer composition 22, which may include adhesive thathas been cured during the manufacturing process, is seen surrounding thespot weld site 40. The interface between strips 10 and 12 is deformed bythe spot weld as seen by the upper curvature of the interlayercomposition 22 adjacent the hardened weld material 46. The hardenedstirred material 46 includes materials from strip 10, strip 12, theapplied powder composition 22, and their reacted products, if any.

Thus, in the example where strip 10 is an aluminum alloy, strip 12 is amagnesium alloy, and the coating material comprises adhesive, copper,tin, and/or zinc, the stir zone 46 includes each of magnesium (and someof its alloying constituents), aluminum (and some of its alloyingconstituents), copper, tin, zinc, and their alloys or compounds (e.g.,Mg₂Sn, Al₃Mg₂) that may be formed during the friction stir process, andany adhesive that was not burned off during the process.

FIG. 4 illustrates abutting strips (or plates or other workpiece shapes)110, 112. Strip 110 is made of a first metal composition and strip 112is formed of a second and dissimilar metal composition. Strips 110, 112have complementary, aligned abutting facing edges between which islocated a layer 122 of coating material for enhancing the formation of astrong friction stir butt weld along the abutting contact surfaces. Thecomposition of coating layer 122 is predetermined by experience orexperiment to provide microstructural constituents to the weld tostrengthen the weld joint between the respective dissimilar compositionsof strips 110 and 112. The coating layer 122 may include adhesive and/ormetallic powders and/or non-metallic powders as described above. Thethickness of coating layer 122 may be of the order of a few tenths of amillimeter to a few millimeters or so and determined to provide asuitable quantity of adhesive and alloying elements or strengtheningconstituents to the butt weld site.

FIG. 5 illustrates the action of a friction stir tool 124 as it isrotated (see rotational directional arrow) and pressed (downwarddirectional arrow) into powder or coating layer 122 and the abuttingedges of strips 110 and 112. In this friction stir welding embodiment,rotating tool 124 is plunged into the abutting top surfaces of theworkpiece strips 110, 112 at the left side edge (as viewed in FIG. 5)and traversed progressively along their interface (traversingdirectional arrow pointing to the right). The adjacent dissimilar metalfaces and interposed powder or coating layer 122 are stirred and mixed.As friction stir tool 124 advances along the facing workpiece faces ahardened stirred material bead 146 is formed that provides a linear seamweld with weld surface 134 between the abutting strips 110, 112. If afull penetration (through thickness) seam weld cannot be achieved bywelding from only one side, then an additional welding operation can bedone from the opposite side. A full penetration weld can also beachieved by using a self-reacting pin tool as known in the openliterature.

The composition of hardened stirred material bead 146 includes elementsof the metal compositions of strip 110, 112 and interfacial coatinglayer 122, including any adhesive that was not burned off during the FSWprocess. The combined compositions provide a stronger weld joint betweenstrips 110 and 112 than is obtained without the use of coatingcomposition 122.

FIG. 6 illustrates an embodiment of the invention in which a linear seamweld is formed between overlapping aluminum alloy and magnesium alloysheets by a friction stir welding process. In this embodiment of theinvention, the use of a coating layer of adhesive and supplementpowdered alloying elements is complemented with the use of a highthermal conductivity supporting anvil to increase the bond strength ofthe friction stir weld.

As illustrated in FIG. 6, a first rectangular aluminum alloy sheet 210has an edge 212 overlying and overlapping edge 214 of a rectangularmagnesium alloy sheet 216. The thickness of sheets 210, 216 may often bein the range from about one-half millimeter to about four millimeters;however, the bottom workpiece 216 may be thicker than four millimeters,when a thick plate, extrusion, or casting is to be part of the frictionstir weld assembly. In this example, sheets 210, 216 are shown to be ofthe same thickness and their thickness is somewhat exaggerated toillustrate the friction stir welding process. Also in this example,edges 212 and 214 are parallel and a linear seam weld is to be formed ina line generally parallel to sheet edges 212, 214 and situated inbetween them.

A coating layer 225 of a composition predetermined to improve thestrength of the lap seam weld was applied to a surface of at least oneof the sheets 210, 216 before they were assembled in the illustratedoverlapping position. In this example, coating layer 225 is applied in agenerally rectangular strip (solid edge and dashed lines in FIG. 6)between the facing surfaces of sheets 210, 216. Coating layer 225extends along the path of the intended linear seam weld. In anembodiment in which the metal parts are formed respectively of analuminum alloy and a magnesium alloy, the coating layer may includemixtures of adhesive and copper-tin, copper-tin-zinc, zinc, or othercompositions such as are described above. In those situations where morethan two sheets are to be welded, interlayer materials such as coatinglayer 225 may be applied to some or all of the faying surfaces. Thecoating compositions may also be different at different faying surfaces,depending on the compositions of two adjacent parent materials. Thissituation applies to both linear friction stir welding and friction stirspot welding processes described above.

Referring again to FIG. 6, the portions to be welded of overlappingsheets 210, 216 are placed on a stack of three rectangular copper alloyanvil plates 218, 220, 222 that, in this example, are the same size andshape. The assembly of overlapping sheets 210, 216 is secured for thefriction stir welding by a suitable fixture or clamping means, notshown. In FIG. 6, the anvil plates 218, 220, 222 extend beyond the edges212, 214 of the sheets 210, 216. In this example, a stack of three anvilplates 218, 220, 222 is employed. However, a single anvil plate, or adifferent number of plates, may be employed to obtain suitable heatdissipation from the friction stir weld site on the thin magnesium andaluminum sheets. Sometimes, for example, greater anvil mass orwater-cooling of the anvil plates is desired when friction stir weldingoperations are continuous and ongoing and the temperature of the anvilmay increase.

A friction stir tool 224 with round cylindrical tool body 226 andtruncated conical end section 228 carrying a profiled probe 230 is usedin making a seam weld. Friction stir tool 224 is gripped in the chuck ofa powered friction stir welding machine, not shown, that rotatesfriction stir tool 224 around a longitudinal axis at the center of roundtool body 226, conical end section 228 and axial probe 230. The frictionstir machine positions friction stir tool 224 over overlapping sheets210, 216 with probe 230 directed nearly perpendicularly at upper surface232 of upper sheet 210. In this example, the friction stir machinerotates friction stir tool 224 as indicated by the curvedcircumferential arrow in FIG. 6 and presses the end of probe 230 againstsurface 232 of aluminum alloy sheet 210 as indicated by the verticalarrow.

As rotating probe 230 of friction stir tool 224 is pressed into sheet210 it plasticizes and stirs the underlying and adjacent aluminum alloyand magnesium alloy sheet material as well as the interposed coatingmaterial layer 225. The friction stir probe 230 penetrates through thethickness of aluminum alloy sheet 210 into magnesium alloy sheet 216. Inthe formation of a seam weld, as is illustrated in FIG. 6, friction stirtool 224 with revolving probe 230 penetrating in the workpiece materialis moved in a linear path generally parallel to sheet edges 212, 214 toprogressively stir and heat the metal and interposed coating layerengaged by friction stir tool 224. As the rotating friction stir tool224 is translated along its predetermined path, the stirred, heated, andmixed sheet metal layers and coating material left behind cools andre-hardens. This re-hardened material is illustrated schematically at234 as a partially formed weld seam. Weld seam 234 comprises mixedelements of the aluminum alloy sheet 210, magnesium alloy sheet 216 andinterposed coating layer 225 to form a composite of the parent materialsand microstructural constituents, such as Mg₂Sn and Al₃Mg₂, formedduring welding, providing a strong weld bead. Weld seam 234,incorporating materials from layer 225 including any adhesive that wasnot burned off during the welding process, is stronger than a seam weldformed only of the original aluminum and magnesium alloy constituents.

In this example, probe 230 penetrates through the thickness of top sheet210 and into underlying sheet 216 to a predetermined depth. After therotating friction stir tool 224 has been moved a predetermined lengthacross the overlapping sheets 210, 216, the linear weld seam 234 extendsacross the width of sheets 210, 216 with the predetermined length.

In this embodiment, a stack of three copper plates 218, 220, 222 areselected to extract excess heat from the friction stir affected regionof the assembly of overlapping sheets to avoid or minimize melting ofthe stir affected material. The thermal conductivity and mass of thethree plates (or a different number or size of plates) is predeterminedby experiment or other analytical means to facilitate friction stirwelding of sheets 210, 216 to obtain the desired performance of the weldand the overlapping sheet assembly.

The above embodiment describes an example of friction stir welding ofaluminum sheet to magnesium sheet with the aluminum sheet being on thetop (i.e., the entry side of friction stir welding tool 224). In thisembodiment high thermal conductivity anvils, such as hard copper alloyor water-cooled steel anvils are used to extract excess heat to maintainadequate temperatures at the welding site to obtain the requiredperformance of the weld and the overlapping sheet assembly.

In another embodiment where the magnesium alloy sheet is on the top andthe aluminum alloy is the bottom work piece in contact with thesupporting anvil, a steel or a less thermally conductive anvil may beused if the heat extraction capability of the aluminum work piececombined with the anvil is excessive such that the required performanceof the weld and the overlapping sheet assembly cannot be obtained. Thissituation applies to both linear friction stir welding and friction stirspot welding processes described above.

Practices of the invention have been described using certainillustrative examples, but the scope of the invention is not limited tosuch illustrative examples.

1. A method of forming a friction stir weld between a magnesium-basedalloy workpiece and an aluminum-based alloy workpiece, the methodcomprising: forming an assembly in which the magnesium-based alloyworkpiece and the aluminum-based alloy workpiece have faying surfaceswith a faying surfaces weld location at which the friction stir weld isto be formed and one of the workpieces has a surface with a frictionstir engagement location for engagement with a rotating friction stirweld tool; placing an interlayer composition at one or both of thefriction stir engagement location or faying surfaces weld location, inwhich the interlayer composition consists essentially of a mixture of anadhesive and one of the materials or material combinations selected fromthe group consisting of: 1) silver, tin, and zinc; 2) silver and tin; 3)copper and tin; 4) zinc; 5) copper, tin, and alumina; 6) alumina; 7)aluminum and alumina; and 8) carbon and tin, as powder; and frictionstirring the magnesium-based alloy workpiece and the aluminum-basedalloy workpiece with a friction stir tool that initially engages thefriction stir engagement location and penetrates the workpieces to eachfaying surfaces weld location; in which the action of the friction stirtool causes mixing of the interlayer composition with metal elements ofthe magnesium-based alloy workpiece and the aluminum-based alloyworkpiece and forms a weld material at the faying surfaces weldlocation, the weld material comprising constituents from the interlayercomposition, each metal workpiece and any reacted products; the weldmaterial forming a stronger friction stir weld bond between themagnesium-based alloy workpiece and aluminum-based alloy workpiece thana friction stir weld bond formed between the workpieces without theplacement of the materials or material combinations; and in which theaction of the friction stir tool also causes at least a portion of theadhesive to cure and to bond the magnesium-based alloy workpiece and thealuminum-based alloy workpiece together.
 2. A method as recited in claim1 in which the assembly is supported on a high thermal conductivitycopper anvil adapted to avoid or minimize melting at the weld location.3. A method as recited in claim 1 in which the assembly of workpieceshas first and second faying surfaces with first and second fayingsurfaces weld locations, and the same interlayer composition is mixedinto the weld material of each of the first and second faying surfaceweld locations.
 4. A method as recited in claim 1 in which the assemblyof workpieces has first and second faying surfaces with first and secondfaying surfaces weld locations, and a first interlayer composition ismixed into the weld material at the first weld location and a second anddifferent interlayer composition is mixed into the weld material at thesecond weld location.
 5. A method as recited in claim 1 in which thecombination of constituents of the interlayer composition with themagnesium and aluminum workpiece constituents increase the meltingtemperature of the weld material.
 6. A method as recited in claim 1 inwhich the combination of constituents of the interlayer composition withthe magnesium and aluminum workpiece constituents increases theviscosity of the weld material.
 7. A method of forming a friction stirweld between a magnesium-based alloy workpiece and an aluminum-basedalloy workpiece, the method comprising: forming an assembly in which themagnesium-based alloy workpiece and the aluminum-based alloy workpiecehave faying surfaces with a faying surfaces weld location at which thefriction stir weld is to be formed and one of the workpieces has asurface with a friction stir engagement location for engagement with arotating friction stir weld tool; placing an adhesive and an interlayercomposition at one or both of the friction stir engagement location orfaying surfaces weld location, in which the interlayer compositionconsists essentially of a mixture of an adhesive with a powder mixtureof tin and one or more of alumina, carbon, copper, silver, and zinc; andfriction stirring the magnesium-based alloy workpiece and thealuminum-based alloy workpiece with a friction stir tool that initiallyengages the friction stir engagement location and penetrates theworkpieces to each faying surfaces weld location, in which the action ofthe friction stir tool causes mixing of the interlayer composition withmetal elements of the magnesium-based alloy workpiece and thealuminum-based alloy workpiece and forms a weld material at the fayingsurfaces weld location, the weld material comprising constituents fromthe interlayer composition, each metal workpiece and any reactedproducts, the weld material forming a stronger friction stir weld bondbetween the magnesium-based alloy workpiece and aluminum-based alloyworkpiece than a friction stir weld bond formed between the workpieceswithout the placement of the tin-containing powder; and in which theaction of the friction stir tool causes at least a portion of theadhesive to cure and to bond the magnesium-based alloy workpiece and thealuminum-based alloy workpiece together.
 8. A method as recited in claim7 in which the assembly is supported on a high thermal conductivitycopper anvil adapted to avoid or minimize melting at the weld location.9. A method as recited in claim 7 in which the assembly of workpieceshas first and second faying surfaces with first and second fayingsurfaces weld locations, and the same interlayer composition is mixedinto the weld material of each of the first and second faying surfaceweld locations.
 10. A method as recited in claim 7 in which the assemblyof workpieces has first and second faying surfaces with first and secondfaying surfaces weld locations, and a first interlayer composition ismixed into the weld material at the first weld location and a second anddifferent interlayer composition is mixed into the weld material at thesecond weld location.
 11. A method as recited in claim 7 in which thecombination of constituents of the interlayer composition with themagnesium and aluminum workpiece constituents increases the meltingtemperature of the weld material.
 12. A method as recited in claim 7 inwhich the combination of constituents of the interlayer composition withthe magnesium and aluminum workpiece constituents increase the viscosityof the weld material.
 13. A method as recited in claim 7 in which thestrength of the weld is at least twice that obtained when the workpiecesare similarly friction stir welded without the interlayer composition.14. A method of forming a friction stir weld between a magnesium-basedalloy workpiece and an aluminum-based alloy workpiece, the methodcomprising: forming an assembly in which the magnesium-based alloyworkpiece and the aluminum-based alloy workpiece have faying surfaceswith a faying surfaces weld location at which the friction stir weld isto be formed and one of the workpieces has a surface with a frictionstir engagement location for engagement with a rotating friction stirweld tool; placing an interlayer composition at one or both of thefriction stir engagement location or faying surfaces weld location, inwhich the interlayer composition consists essentially of a mixture of anadhesive and one or more of the materials selected from the groupconsisting of alumina, aluminum, carbon, copper, silver, tin, and zinc,as powder; and friction stirring the magnesium-based alloy workpiece andthe aluminum-based alloy workpiece with a friction stir tool thatinitially engages the friction stir engagement location and penetratesthe workpieces to each faying surfaces weld location; in which theaction of the friction stir tool causes mixing of the interlayercomposition with metal elements of the magnesium-based alloy workpieceand the aluminum-based alloy workpiece and forms a weld material at thefaying surfaces weld location, the weld material comprising constituentsfrom the interlayer composition, each metal workpiece and any reactedproducts; the weld material being composed to form a stronger frictionstir weld bond between the magnesium-based alloy workpiece andaluminum-based alloy workpiece than a friction stir weld bond formedbetween the workpieces without the placement of the powder materials;and in which the action of the friction stir tool also causes at least aportion of the adhesive to cure and to bond the magnesium-based alloyworkpiece and the aluminum-based alloy workpiece together.
 15. A methodas recited in claim 14 in which the assembly is supported on a highthermal conductivity copper anvil adapted to avoid or minimize meltingat the weld location.
 16. A method as recited in claim 14 in which theassembly of workpieces has first and second faying surfaces with firstand second faying surfaces weld locations, and the same interlayercomposition is mixed into the weld material of each of the first andsecond faying surface weld locations.
 17. A method as recited in claim14 in which the assembly of workpieces has first and second fayingsurfaces with first and second faying surfaces weld locations, and afirst interlayer composition is mixed into the weld material at thefirst weld location and a second and different interlayer composition ismixed into the weld material at the second weld location.
 18. A methodas recited in claim 14 in which the combination of constituents of theinterlayer composition with the magnesium and aluminum workpiececonstituents increases the melting temperature of the weld material. 19.A method as recited in claim 14 in which the combination of constituentsof the interlayer composition with the magnesium and aluminum workpiececonstituents increase the viscosity of the weld material.
 20. A methodas recited in claim 14 in which the strength of the weld is at leasttwice that obtained when the workpieces are similarly friction stirwelded without the interlayer composition.